In a typical wireless communication network, wireless devices, also known as wireless communication devices, mobile stations, stations (STA) and/or user equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks. The RAN covers a geographical area which is divided into service areas or cell areas, with each service area or cell area being served by a radio network node such as a radio access node e.g., a Wi-Fi access point or a radio base station (RBS), which in some networks may also be denoted, for example, a “NodeB” or “eNodeB”. The service area or cell area is a geographical area where radio coverage is provided by the radio access node. The radio access node communicates over an air interface operating on radio frequencies with the wireless device within range of the radio access node.
A Universal Mobile Telecommunications System (UMTS) is a third generation telecommunication network, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some RANs, e.g. as in UMTS, several radio access nodes may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural radio access nodes connected thereto. This type of connection is sometimes referred to as backhaul connection. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS) have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access network, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio access nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of an RNC are distributed between the radio access nodes, e.g. eNodeBs in LTE, and the core network. As such, the Radio Access Network (RAN) of an EPS has an essentially “flat” architecture comprising radio access nodes connected directly to one or more core networks, i.e. they are not connected to RNCs. To compensate for that, the E-UTRAN specification defines a direct interface between the radio access nodes, this interface being denoted the X2 interface.
Of the upcoming fifth generation of wireless communication networks 5G, one key design principle currently under consideration is to base the wireless communication network on an ultra-lean design. This implies that “always on signals” from the network should be avoided as much as possible. The expected benefits from this design principle are that the wireless communication network should have a significantly lower network energy consumption, a better scalability, a higher degree of forward compatibility during the radio access technology (RAT) evolution phase, a lower interference from system overhead signals and consequently a higher throughput in low load scenario, and an improved support for user centric beam-forming.
There are principally two sets of mobility procedures considered in both the current LTE standard as well as in the ongoing 5G discussions.
The first one is denoted ‘Idle Mode Mobility’ and defines how a wireless device which is deemed ‘Idle’, i.e. the wireless device has no ongoing nor recent data transfer, shall be able to reach the wireless communication network using random access procedures and how to be reachable from the wireless communication network by means of paging procedures etc. In idle mode, the mobility procedures, e.g. handovers or cell selections, are typically controlled by the wireless device based on a set of rules, e.g. signal level thresholds and carrier frequency priorities, decided by the wireless communication network.
The other one is ‘Active Mode Mobility’, which has a main task of maintaining the connectivity for an ‘Active’ or ‘Connected’ wireless device, i.e. the wireless device actually has an ongoing or recent data transfer, as the wireless device moves around in the wireless communication network, and also to handle abnormal cases such as failed handovers, radio link failures etc. In ‘Active Mode Mobility’ the mobility procedures are typically controlled by the wireless communication network, potentially based on measurements from the wireless device.
A complete X2-based intra-Mobility Management Entity (MME)/intra Serving Gateway (S-GW) Handover (HO) procedure for an LTE system is given in 3GPP TS 36.300 “E-UTRA(N) Overall Description; Stage 2” version: V12.4.0 (2014-12).
A key difference between the current LTE procedures as per above, and the upcoming 5G procedures, is that in an ultra-lean system as 5G, as described above, the radio network nodes will prevent themselves from keeping some of the ‘always-on’ signal unlike their counter parts in the LTE system. Instead, the wireless communication network needs to activate the necessary reference signals/beams to measure on only when needed.
The term ‘beam’ used herein is defined in relation with a reference signal (RS). That is, from the wireless device's standpoint a beam is considered as an entity that the wireless device may associate with and is recognized via some reference signals specific to that beam which, in the case of a legacy LTE network may be the Cell-specific Reference Signals (CRS) of the cell or wireless device specific reference signals for a specific wireless device. In a wireless communication network with more than one antenna, it is possible for the wireless communication network to form directive antenna radiation patterns, a process which is most often related to as ‘beam-forming’. In future wireless communication systems with a large number of antennas, this beam-forming may be very directive and hence provide a very high antenna beamforming gain. In such beam-forming cases, there may be other types of reference signals present, here called simply Beam Reference Signals (BRS) or Mobility Reference Signals (MRS). In all essence however, regardless of the level of directivity of the formed antenna pattern, it is still considered a ‘beam’. Hence, for the simplicity of the exposition, the term ‘beam’ will be used herein.
A service area of a radio network node is a region surrounding the radio network node in which the radio network node is responsible for the active mode mobility related measurements from the wireless device. A wireless device outside such a service area could still be served by the beams from the radio network node but a neighbor radio network node providing radio coverage will be ideally suited for mobility related aspects for the wireless device. Also, such a service area could be a virtual region or could be defined by certain reference signals' coverage. Hence, this 5G concept of service area could be resembled to the coverage area/cell concept of a current LTE system, which has no counterpart in a massively beam-formed system without cell-specific reference symbols being always on.
The mobility procedures also referred to as the handover or cell selection procedures as described above may be refined further in a scenario where it is not certain that a potential target eNB is transmitting the relevant reference signals corresponding e.g. to a given beam, which is assumed in the legacy case above. In such case, a request to start transmitting these reference signals is required, which could e.g. as per FIG. 1, where a serving eNB, eNB1, at an early stage, based on some logic not shown here, requests, with a reference signal request, a potential target eNB, eNB2, to start transmitting one or more RSs or beams that can be used for HO related measurements by the wireless device to support the HO procedure, see action 1a. The eNB2 starts the RSs or beams, action 1b in the FIG. 2. Action 1c. The eNB1 sends the wireless device, W1, some measurement control information for enabling measurements. Action 2. The wireless device W1 reports back to the eNB1 with measurement reports. Action 3. The eNB1 makes a HO decision based on the received measurement reports. In case a HO is decided, the eNB1 transmits a handover request to the eNB2, see Action 4. Action 5. The eNB2 performs an admission control and in case the admission control is successful, the eNB2 sends a handover request acknowledgement (Ack) to the eNB1, see Action 6.
The underlying assumption here is that the serving eNB, eNB1, keeps track of which reference signals, i.e. beams, that are to be started in action 1b in FIG. 1 above based on a position of a considered wireless device together with a lookup in a position-to-beam mapping table containing beams also in other radio network nodes than itself. The end result being that the eNB1 will indicate explicitly to the eNB2 exactly which reference signals/beams it wants the eNB2 to start transmitting.
Now, keeping this position-to-beam mapping table in an optimal state—i.e. where exactly those beams that are usable for a given position, but no others, are mapped to that position—is rather difficult. If not done properly, it may end up so that too many beams are started resulting in a waste of resources and extra interference, or too few beams are started resulting in a failure to find the best beam and risk ending up in a sub-optimal allocation. Furthermore, it could be quite complex to establish an initial position-to-beam mapping table for the initial times—especially with regards to entries in the position-to-beam mapping table relating to other radio network nodes. Also, it could be so that there are time variations on which beams are best at a given position, hence this position-to-beam mapping table would need another dimension, e.g. time, which would make such a solution even more complex. Thus, a problem with present solutions is that resources may be wasted or that a beam that is not the optimal may be used resulting in a limited performance of the wireless communication network.